1. Field of the Invention
The present invention relates to a blood filter system for use in connection with the filtration of blood in medical devices such as but not limited to extracorproreal circulation systems used during cardiopulmonary support Procedures. The blood filter system has particular utility in connection with the sequential filtering of blood while actively removing air bubbles and debris.
2. Description of the Prior Art
Blood filter systems for extracorporeal circulations are desirable for eliminating micro-bubble contamination and foreign particles from the patient's systemic circulation. It is critical that during any cardiopulmonary support procedures, there are no free gases, or air bubbles present in the blood returning to the patient. It is also critical that blood contacts the least amount of foreign surfaces and contains the least amount of dilution from the extracorporeal circulation system.
The use of blood filters is known in the prior art. For example: U.S. Pat. Nos. 4,572,724; 5,744,047; 5,540,841; 5,362,406; 5,258,127, and 3,701,433.
While the above-described devices fulfill their respective objectives and requirements, the aforementioned patents do not describe a blood filter system that allows the sequential filtering of blood while actively removing air bubbles and debris.
Accordingly, the prior art for blood filter devices used in extracorporeal circulation systems utilizes a large volume chamber with an inlet and outlet, a single stage micro-pore screen filter placed at 90 degree angles to the blood flow path and a large contact surface area to filter air bubbles and debris from the blood stream.
The large volume chamber provides a decrease in blood flow velocity as blood passes through the filter to allow more time for the buoyant forces of any air bubbles present to act, causing them to rise up and separate from the blood stream.
Additionally, construction of the large volume chamber usually includes rounded walls that force the blood to flow in a circular motion to create centrifugal forces within the filter chamber. The centrifugal forces acting on the various elements of blood cause the more dense Red Blood Cell's to spread out along the filters outer wall, while less dense particles such as air bubbles, accumulate at the chambers center where a purge port is located for easy removal.
Moreover, placement of the single stage micro-pore screen filter at 90 degree angles to the blood flow path as described in the prior art reduces filter efficiency and debris clearance from the filter housing. As air bubbles and debris strike the pleated single stage micro-pore screen head on at 90 degree angles to its flow path, they can become effectively trapped between the pleats and held against the screen by the force of the blood flowing through it. Also, the force of collision caused by the direct impact between air bubbles and the filter medium placed at right angles to its flow path promotes further micro-bubble generation as larger bubbles colliding with the screen break apart and increase the amount of filtration required.
Finally, extracorporeal blood filters rely heavily on the principle that describes the Bubble Point Pressure as a means to separate air bubbles from the blood stream. The Bubble Point Pressure is defined as the amount of pressure required to eject air across a wetted pore. As air bubbles cannot easily pass through a wetted pore, their passage is blocked by the filter screen, which separates them from blood flow exiting the filter unless there exists a sufficiently high pre-screen pressure to force them through. Air bubbles entering the filter accumulate on the proximal surface of the micro-pore screen and obstruct blood flow so as to cause a rise in pre-screen pressure within the filter housing. As air bubble accumulation on the micro-pore screen continues, the pre-screen pressure also continues to rise until such time that the Bubble Point Pressure is reached and air is ejected across the wetted pore.
To avoid this, currently available blood filter systems make use of a large micro-pore screen surface area to increase the number of pores available and thereby reduce the potential risk of reaching the Bubble Point Pressure. Increasing the number of available pores is usually accomplished by tightly folding, or pleating extra screen filter material into the filter housing. This large screen surface area contributes negatively to blood handling as it constitutes the majority of foreign surface available for blood contact activation and also sets the requirement for a larger volume filter housing. Although the addition of extra screen filter material helps reduce the possibility of reaching the Bubble Point Pressure, it also makes the job of ensuring the filter is properly de-aired in a timely fashion during priming more difficult, creating a possible safety hazard for the patient.
In conclusion, the above mentioned disadvantages of the prior art are counterproductive to blood filtration as they may promote the build-up and retention of air bubbles and debris on the micro-pore screen, reduce filter efficiency and exposes the patient to increased risk of embolization.
The currently available single stage micro-pore screen filter as described in the prior art is unable to effectively remove all air bubbles and debris from the extracorporeal blood flow. In addition to the foreign surface contact activation and excessive dilution commonly seen during extracorporeal circulations, inadequate filtration continues to play a major role in the problems associated with cardiopulmonary support procedures. Furthermore, advancements in the science of extracorporeal circulation will likely continue to increase demand on system components and their performance as a means of achieving improved patient outcomes. Therefore, a need exists for an improved blood filter system that can increase filter efficiency while decreasing the amount of foreign contact surface area and priming volume required. In this regard, the present invention while departing from conventional concepts and designs of the prior art, substantially fulfills this need and provides an apparatus primarily developed for the sequential filtering of blood and active removal of air bubbles and debris during extracorporeal circulations.